Oxide film forming method

ABSTRACT

To provide a method for the formation of oxide films to form with advantage a high-quality oxide film having excellent uniformity in film thickness and film quality over the entire wafer. The method for the formation of oxide films comprises: the pretreatment process of forming a protective oxide film on the surface of a wafer positioned in a reaction vessel by performing oxidation treatment with radical oxidative species or an atmosphere containing radical oxidative species under depressurized conditions; and the oxide-film-formation process of forming an oxide film on the wafer by performing oxidation treatment at a predetermined temperature under depressurized conditions. The oxide-film-formation process is preferably performed following the pretreatment process in a continuous manner in the reaction vessel in which the pretreatment process is performed. The pretreatment process is preferably performed at a temperature lower than the temperature for the oxide-film-formation process and also preferably performed under depressurized conditions, the level of the depressurization being higher than the level for the oxide-film-formation process. A high-quality gate-insulating film for a transistor chip can be formed according to this method for the formation of oxide films.

TECHNICAL FIELD

[0001] The present invention relates to a method for forming an oxidefilm on the surface of a wafer composed of silicon for example.

BACKGROUND ART

[0002] In manufacturing of semiconductor devices, various kinds ofprocessing are done on semiconductor wafers consisting of silicon or thelike, and the processing includes oxidization processing to form anoxide film on the surface of the wafer. The formed oxide film functionsas a gate-insulating film of transistor chip, for example. Thegate-insulating films are needed in recent years to be thin in filmthickness together with long-running high reliability.

[0003] Generally, the oxide film formation on a wafer is processed byplacing a wafer inside a reaction vessel heated to a high temperature,e.g. 750 to 1100° C., in the presence of oxygen under the conditions ofnormal or reduced pressure. And the oxide film formation method underdepressurized conditions particularly has an advantage that thegenerating rate of oxide film is small due to the low oxygenconcentration whereby the more degrees of freedom for controlling theformation of oxide film is easy even if thin film thickness is formed.

[0004] However, in such a oxide film formation process in a low oxygenatmosphere at a high temperature, the problem is that the oxide filmsobtained in the oxide film form process turn out to have low reliabilitydue to the roughness formed on the surface of the wafer by being etchedor nitriding by the nitrogen gas for generating the atmosphere of lowoxygen concentration during the time period from loading of the waferinto the reaction vessel to the initiation of actual oxide filmformation, i.e. during the heating process to raise the temperature ofthe reaction vessel or the surface of the wafer to a predeterminedtemperature for the oxidation treatment.

[0005] In particular, it is difficult to form high-quality oxide filmswith high uniformity of the film thickness and film quality over theentire area of larger-diameter wafers which are common now.

[0006] Given such situations, the following methods are employed to formhigh-quality oxide films while averting the problems caused by the oxidefilm formation process in a low oxygen atmosphere at a high temperature.

[0007] (1) A method of forming an initial oxide film on the surface of awafer as a protective oxide film by maintaining the oxidation atmosphereeven in the heating process when carrying out the oxide film formationprocess at normal pressure.

[0008] In this method, improved uniformity of an oxide film can beobtained in the oxide film formation process because the protectiveoxide film is formed on a wafer under somewhat controlled condition inthe heating process.

[0009] (2) A method of forming a protective oxide film in advance on thesurface of a wafer by chemical treatments such as a wet cleaning withhydrogen peroxide solution etc. for preventing oxide films to have lowreliability because of the low oxygen atmosphere at a high temperaturewhen carrying out the oxide film formation process at normal pressure.

[0010] In this method, degradation of reliability of the oxide filmformed in the oxide film formation process becomes controllable becausethe protective oxide film is formed under controlled conditions. Thismethod is advantageous when raising temperature in a low oxygenatmosphere in order to prevent the film thickness of a protective oxidefilm from being thick.

[0011] (3) A method of forming a protective oxide film with controlledfilm thickness by controlling the partial pressure of oxygen inside areaction vessel in the heating process when carrying out the oxide filmformation process at reduced pressure.

[0012] (4) A method of forming a protective oxide film with controlledfilm thickness by controlling the partial pressure of oxygen in theheating process by dilution with an inert gas as such as a nitrogen gaswhen carrying out the oxide film formation process at normal pressure.

[0013] However, the above methods have the following problems:

[0014] (1) In the method of positively forming a protective oxide filmby performing the heating process in the oxidation atmosphere,anticipating the film thickness of the protective oxide film formed inthe heating process, which is necessary to accomplish the process of theoxide film formation, is quite difficult since the film thickness of theprotective oxide film is difficult to be controlled. For instance, in acase that an ultrathin oxide film with the final film thickness of 2 nmor less needs to be formed in particular, the operating condition wouldhave much less variation in the oxide film formation process.

[0015] Although the protective oxide film formed in this way isgenerally considered to have a poor film quality, the protective oxidefilm may be reformed in the process of oxide film formation in manycases. However, in a case that an oxide film with the final filmthickness of 2 nm or less is formed in particular, forming an oxide filmwith high film quality would turn out to be difficult due to thedifficulty of reforming the protective oxide film sufficiently in theoxide film formation process because the protective oxide film occupiesa large portion of the oxide film finally obtained.

[0016] Furthermore, in general, a protective oxide film is formed insidea reaction vessel or on the surface of a wafer where the temperaturedistribution is uneven, therefore, the oxide film finally formed wouldturn out to provide quite poor uniformity in film thickness since thefilm thickness distribution of the protective oxide film formed on thesurface of a wafer would show a significant nonuniformity correspondingto a nonuniformity of the temperature of the surface of the wafer.

[0017] (2) In the method of forming a protective oxide film usingchemical means by a wet cleaning, a slight amount of metallic elementswould inevitably be introduced into the chemically formed protectiveoxide film in the process of the wet cleaning, which fact is the causeof low reliability of the oxide film finally formed.

[0018] This method also have a problem that organic contaminants arelikely to adhere to the surface of a wafer since the wafer has anoccasion to be exposed to atmospheric air before the wafer is broughtinto a reaction vessel after the protective oxide film is formed by awet cleaning. Although, in some cases, the organic matters adhered tothe surface of the wafer can be removed during combustion by executingthe heating process in an oxidation atmosphere, uniformity and controlof the film thickness of the oxide film finally formed wouldsignificantly be degraded and thus the method is not preferable.

[0019] Moreover, the protective oxide film formed using chemical meansby a wet cleaning has low reliability in uniformity in film thicknesswithin the entire surface of a wafer, in controllability of filmthickness and in film quality, and the oxide film finally formed wouldhave low reliability as a result.

[0020] (3) The method of forming a protective oxide film in the heatingprocess at reduced pressure is implemented while the partial pressure ofoxygen is being lowered, and the protective oxide film itself can bethinner compared to methods implemented at normal pressure. However,although greater degree of freedom can be obtained to control the finalfilm thickness, the problem is that the entire film thickness would benonuniform since nonuniformity of the wafer surface temperature affectson thickness distribution of the protective oxide film which is formedin the heating period.

[0021] (4) In the method of forming a protective oxide film withcontrolled film thickness by controlling the partial pressure of oxygenin the heating process with a nitrogen gas, forming a high-quality oxidefilm is difficult as stated previously due to a wafer inevitablynitrided.

[0022] As stated above, the problem is that a high-quality oxide filmcannot be formed with advantage on a wafer by the conventional oxidefilm formation methods.

[0023] The present invention suits to solve the problems stated above,and the purpose of the present invention is to provide an oxide filmformation method to form with advantage a high-quality oxide film havingexcellent uniformity in film thickness and film quality over the entirewafer.

[0024] The present invention has been completed based on the knowledgeobtained by locating the root cause of the low reliability of an oxidefilm formed in the oxide film formation process, and the root cause isthat a stable protective oxide film cannot be formed under conditionswith sufficient control before starting the process of oxide filmformation by the conventional oxide film formation methods.

DISCLOSURE OF INVENTION

[0025] The oxide film formation method of the present invention ischaracterized by comprising: a pretreatment process of forming aprotective oxide film on the surface of a wafer positioned in a reactionvessel, the process being performed by oxidation treatment with radicaloxidative species under depressurized conditions or by oxidationtreatment with an atmosphere containing radical oxidative species underdepressurized conditions and a oxide film formation process to form anoxide film on the surface of said wafer with the protective oxide filmobtained by the pretreatment process, the process being performed byoxidation treatment at a predetermined temperature under depressurizedconditions.

[0026] In the above-mentioned method for the formation of oxide films,the oxide film formation process is preferably performed following thepretreatment process in a continuous manner in said reaction vessel inwhich said pretreatment process is performed.

[0027] The pretreatment process is preferably performed at a temperaturelower than the temperature for the oxide-film-formation process, andalso is preferably performed under depressurized conditions, the levelof the depressurization being higher than the level for the oxide filmformation process.

[0028] The oxidation treatment in the pretreatment process is preferablyperformed under the conditions that the temperature is from 25 to 600°C. and the pressure is from 13.3 to 101080 Pa (0.1 to 760 Torr).

[0029] The radical oxidative species used for the oxidation treatment inthe pretreatment process are preferably produced by ozone or composed ofO* (oxygen radical species) and OH* (hydroxyl radical species).

[0030] Also, the wafer heating operation in the pretreatment processpreceded to the oxide film formation process is preferably performed ata temperature from 750 to 1100° C. in an atmosphere with a partialpressure of oxygen of 1000 Pa or less.

[0031] Moreover, the pressure is preferably controlled to be from 13.3to 101080 Pa (0.1 to 760 Torr) and the partial pressure of oxygen ispreferably controlled to be 1000 Pa or less in the pretreatment process.

[0032] Furthermore, the oxidation treatment in the oxide film formationprocess can be performed by the means selected from a wet oxidationmethod, a dry oxidation method, an internal combustion method, anoxidation method with radical oxidative species and an oxidation methodwith an atmosphere containing hydrogen chloride, under the conditionsthat the pressure is from 133 to 101080 Pa (1 to 760 Torr) and thetemperature is from 750 to 1100° C.

[0033] A gate-insulating film of transistor chip of the presentinvention is characterized in that the film is formed according to theabove-mentioned oxide film formation method.

[0034] Also, the method for forming gate-insulating films of transistorchips is characterized by performing the above-mentioned oxide filmformation method.

[0035] According to the oxide film formation method of the presentinvention, a high-quality oxide film having excellent uniformity in filmthickness and film quality over the entire surface of a wafer can beformed in the oxide film formation process since a protective oxide filmis formed on the surface of the wafer positioned inside a reactionvessel by performing oxidation treatment with radical oxidative species(including oxidation treatment with an atmosphere containing radicaloxidative species) under depressurized conditions.

[0036] This is because the formed protective oxide film is an oxide filmhaving sufficient uniformity in film thickness with high film quality toguarantee the beneficial effects that a spontaneous oxidation of a waferin uncontrolled state before the initiation of the oxide film formationprocess can be effectively prevented, and as a result, a quality oxidefilm with high uniformity as a whole can be formed completely withadvantage even in a case that the film thickness of the entire oxidefilms is thin.

[0037] A predetermined processing can hence be accomplished with ratherhigh efficiency by performing the oxide film formation process followingthe pretreatment process in a continuous manner in the reaction vesselin which said pretreatment process is performed.

[0038] Also, in the pretreatment process, the effects stated above canassuredly be achieved by performing said pretreatment process in atemperature lower than the temperature for the oxide film formationprocess or under depressurized conditions, the level of thedepressurization being higher than the level for the oxide filmformation process.

[0039] In the pretreatment process, the above-mentioned effects canassuredly be achieved by performing the oxidation treatment under theconditions that the pressure is 13.3 to 101080 Pa (0.1 to 760 Torr) andthe temperature is 25 to 600° C., and by the radical oxidative speciesproduced by ozone or composed of O* (oxygen radical species) and OH*(hydroxyl radical species), said radical oxidative species being usedfor the oxidation treatment particularly in the pretreatment process.

[0040] Moreover, the effects stated above can assuredly be achieved byperforming the wafer heating operation preceded to the oxide filmformation process at a temperature from 750 to 1100° C. in an atmospherewith a partial pressure of oxygen of 1000 Pa or less in the pretreatmentprocess.

[0041] Furthermore, the effects stated above can assuredly be achievedby controlling the pressure to be 13.3 to 101080 Pa (0.1 to 760 Torr)and the partial pressure of oxygen to be 1000 Pa or less in thepretreatment process.

[0042] Desired effects can be achieved by the oxidation treatment in theoxide film formation process by any method among a wet oxidation method,a dry oxidation method, an internal combustion method, an oxidationmethod with an atmosphere containing radical oxidative species and anoxidation method with an atmosphere containing hydrogen chloride, underthe conditions that the pressure is from 133 to 101080 Pa (1 to 760Torr) and the temperature is from 750 to 1100° C.

[0043] In addition, by forming a gate-insulating film of a transistorchip in particular according to the above oxide film formation method, agate-insulating film with favorable characteristics and long life can beobtained.

BRIEF DESCRIPTION OF DRAWINGS

[0044]FIG. 1 is explanatory drawings showing the configuration of anexample of a batch-operating vertical apparatus for oxidation treatmentemployed in the oxide film formation method according to the presentinvention.

[0045]FIG. 2 is a sequence diagram schematically showing a specificprocess of the oxide film formation method according to the presentinvention.

[0046]FIG. 3 is a sequence diagram showing a process of an embodiment ofthe oxide film formation method according to the present invention.

[0047]FIG. 4 is a graph showing the outcome of variation in the leakagecurrent of the oxide film according to the embodiment of the presentinvention along with the outcome of a control sample.

[0048]FIG. 5 is a graph showing the Weibull plot of constant voltageTDDB according to the embodiment of the present invention along with theoutcome of a control sample.

BEST MODE FOR CARRYING OUT THE INVENTION

[0049] Hereinafter, the present invention will be explained in detailwith reference to the drawings.

[0050]FIG. 1 is explanatory drawings showing the configuration of anexample of a batch-operating vertical apparatus for oxidation treatmentemployed in the oxide film formation method according to the presentinvention.

[0051] The oxidation treatment apparatus 10 comprises a reaction vessel12 which is cylindrical and vertically extended, has a single tubularstructure closed at the top thereof and is composed of silicon dioxideor the likes.

[0052] Below the reaction vessel 12, a manifold 13 is positioned whichis a cylindrical stainless steel and tightly joined to the lower end ofsaid reaction vessel 12. Below the manifold 13, a cover member 14 ispositioned which can be shifted up and down by a boat elevator (notshown).

[0053] In this way by lifting the cover member 14 to close the openingof the lower end of the manifold 13, a sealed reaction process chamberPC is formed inside the reaction vessel 12.

[0054] On the cover member 14A, a wafer boat 15 which is composed ofsilicon dioxide for example is mounted. A plurality of the semiconductorwafers 16 consists of silicon for example, on each of which oxide filmis to be formed, are held at regular intervals in a vertical directionon the wafer boat 15. By lifting the cover member 14 with the boatelevator to insert the wafer boat 15 into the reaction vessel 12, thewafers 16 held on said wafer boat 15 are positioned in a processing areaPA inside the reaction vessel 12.

[0055] The reaction vessel 12 is about the size that a cylindrical spaceD is to be defined between the inner surface of the reaction vessel 12and the wafer boat 15 or the outer circumferential rim of the wafer 16after the wafer boat 15 is inserted into the reaction vessel 12. Thiscylindrical space D is configured to have a size of approximately 20 to50 mm for example to obtain desired exhaust conductance within saidreaction vessel 12 considering after-mentioned gas flow rate andpressure inside the reaction vessel 12.

[0056] A warm-up heater 17 comprising a resistance heating element isprovided around the reaction vessel 12 to encircle said reaction vessel12, and this warm-up heater 17 applies heat for the temperature insidethe reaction vessel 12 or the temperature of the positioned wafer 16 toreach a predetermined preset temperature.

[0057] Meanwhile, in the manifold 13, a processing gas supply pipe 18 isprovided piercing through the peripheral wall of said manifold 13. As apipe material of this processing gas supply pipe 18, Teflon is used forits resistance to corrosion. An end portion 18 a of this processing gassupply pipe 18 which is positioned inside the manifold 13 includes aprocessing gas induction part 18 b which is bended and extended upward,and the opening at the end of the processing gas induction part 18 b ispositioned to face upward at the lower end of the cylindrical space D ofthe reaction vessel 12.

[0058] As a result, through this processing gas induction part 18 b, aprocessing gas is basically supplied upward toward the cylindrical spaceD and then, by diffusion upon reaching the upper end (ceiling) of thereaction vessel 12 for example, supplied to the processing area PA wherethe wafer 16 is positioned.

[0059] Connected to the processing gas supply pipe 18 is an ozonegenerator 19. The ozone generator 19 in this example is equipped with aplasma generator for example and forms ozone from oxygen. Connected tothe ozone generator 19 in this example is a purifier 20 to which anoxygen gas supply pipe 21 and an additive gas supply pipe 22 areconnected.

[0060] With the purifier 20, the oxygen gas from the oxygen gas supplypipe 21 and the additive gas, in the form of a nitrogen gas or a carbondioxide gas, from the additive gas supply pipe 22 have suitableconditions for forming ozone in terms of purity. To be more precise,gases are supplied to the ozone generator 19 in the conditions which aresatisfied in reducing the production of impurities, a corrosive gas bymoisture in particular.

[0061] Also, a vent 23 is provided on the manifold 13 at the oppositeside of where the processing gas supply pipe 18 pierces through, and anexhaust pipe 24 is connected to the vent 23, and a vacuum pump 26 isconnected to the exhaust pipe 24 with a combination valve 25 insertedtherebetween.

[0062] The vacuum pump 26 exhausts the reaction vessel 12 of gas throughthe exhaust pipe 24 and keeps the reaction vessel 12 depressurizedinside, and the combination valve 25 functions by adjusting the openingthereof to control the pressure inside the reaction vessel 12.

[0063] A nitrogen gas supply pipe 27 is connected to the manifold 13 inthe lower position of the vent 23 to supply a nitrogen gas for example.

[0064] Moreover, a second processing gas supply pipe 32 is provided inthe manifold 13. In the same manner as the processing gas supply pipe18, the second processing gas supply pipe 32 may have an end portion 32awhich is bended an extended upward, and the opening of the end ispositioned to face upward at the lower end of the cylindrical space D ofthe reaction vessel 12 along with the processing gas induction part 18b.

[0065] The second processing gas supply pipe 32 supplies processinggases as necessary for various kinds of processing and is used as anoxygen gas supply pipe, for example.

[0066] In addition, a control mechanism (not shown) is provided tocontrol the operating conditions of each of the above-mentioned ozonegenerator 19, purifier 20, oxygen gas supply pipe 21, additive gassupply pipe 22, combination valve 25, vacuum pump 26, nitrogen gassupply pipe 27, second processing gas supply pipe 32, etc. To be moreprecise, this control mechanism is equipped with a microprocessor, aprocess controller, etc. and functions to measure the parameters of theprocessing condition of the oxidation treatment apparatus 10 such as thetemperature and pressure of each section and send control signals toeach section accordingly based on the measurement data.

[0067] According to the present invention, an oxide film is formed onthe surface of a wafer by processing the wafer as described below withthe oxidation treatment apparatus 10 having the above-mentionedstructure.

[0068]FIG. 2 is a sequence diagram schematically showing a specificprocess of the method for the formation of oxide films according to thepresent invention. As shown in this diagram, the method for theformation of oxide films according to the present invention has threesteps consisting of a precleaning process as the first step, apretreatment process from loading down to the third step as the secondstep, and an oxide film formation step (hereinafter referred to as a“film formation process”) as the third step.

[0069] [Precleaning Process]

[0070] This precleaning process is a process of cleaning the surface ofthe wafer on which an oxide film is to be formed. To be precise, thesurface of the wafer is cleaned by being immersed in a hydrofluoric acidsolution of low concentration, for example, to mainly remove the oxidesformed on the surface of the wafer.

[0071] Although the specific parameters in this precleaning process arenot necessarily limited, the temperature may be 23° C. and theconcentration of the chemical liquid may be 1 vol. % in the process, forexample.

[0072] Other cleaning means or a combination of proper cleaning meanscan be employed instead in the precleaning process.

[0073] [Pretreatment Process]

[0074] The pretreatment process has 4 steps including:

[0075] (1) a loading operation in which a wafer is loaded to bepositioned in the reaction vessel of the oxidation treatment apparatus;

[0076] (2) an adjusting-processing-parameter operation to maintaininside the reaction vessel at the predetermined pressure andtemperature;

[0077] (3) a protective-oxide-film-forming operation; and

[0078] (4) a heating operation.

[0079] (1) Loading Operation

[0080] In the loading operation, the subject wafer is loaded into thereaction vessel of the oxidation treatment apparatus and positioned inthe processing area PA.

[0081] In more specific terms referring to the oxidation treatmentapparatus 10 of FIG. 1, the wafer boat 15 holding the wafer 16 afterbeing cleaned in the precleaning process is mounted on the cover member14 staying down.

[0082] Meanwhile, the temperature inside the reaction process chamber PCis controlled at an established predetermined loading temperature of t1by heat application with the warm-up heater 17 as necessary. Thetemperature t1 for loading ranges from 25° C. (a room temperature) to600° C., for example.

[0083] The temperature t1 for loading preferably is within the rangefrom 50 to 550° C. and 200 to 400° C. in particular, depending on thekinds or conditions of the means for forming protective oxide filmsactually employed.

[0084] Under these conditions, the wafer boat 15 is loaded into thereaction vessel 12 by lifting the cover member 14 using the boatelevator, not shown, thereby positioning the wafer 16 in the processingarea PA inside the reaction vessel 12 (loading), and at the sameinstant, the reaction process chamber PC is sealed because the openingof the lower end of the- manifold 13 is closed with the cover member 14.

[0085] (2) Adjusting-Processing-Parameter Operation

[0086] The adjusting-processing-parameter operation is performedfollowing the above-mentioned loading operation to suitably conditioninside the reaction process chamber PC for theprotective-oxide-film-forming operation.

[0087] To be precise, the operation is performed to provide anatmosphere of reduced pressure inside the reaction process chamber PC.

[0088] Gas is exhausted from the reaction process chamber PC until thepressure inside the reaction vessel 12 reaches the pressure level of thepredetermined reduced pressure, e.g. 13.3 to 26600 Pa (0.1 to 200 Torr),from the normal pressure of 101080 Pa (760 Torr).

[0089] Concurrently with the above-mentioned depressurization operation,the temperature inside the reaction process chamber PC is beingcontrolled to have the established predetermined temperature t2 forforming protective oxide films by heat application with the warm-upheater 17 as necessary. In this operation, the targeted value of thetemperature inside the reaction process chamber PC is the temperature t2for forming protective oxide films in the followingprotective-oxide-film-forming operation.

[0090] The above depressurization operation and heating operation arerun for the necessary time period, e.g. approximately 10 to 20 minutes,for the state of the predetermined pressure and temperature inside thereaction process chamber PC to be stable so that the condition insidethe reaction process chamber PC is stabilized.

[0091] (3) Protective-Oxide-Film-Forming Operation

[0092] This operation is performed to form a protective oxide film onthe surface of a wafer under the conditions that the pressure andtemperature inside the reaction process chamber PC is stable asdescribed above.

[0093] To be more precise, the supply of a nitrogen gas from thenitrogen gas supply pipe 27 is ceased and an oxygen gas of apredetermined amount is supplied to the purifier 20 from the oxygen gassupply pipe 21 at a flow rate of 1 to 10 slm for example, andconcurrently a nitrogen gas of a predetermined amount is supplied to thepurifier 20 from the additive gas supply pipe 22 at a flow rate of 0.008to 0.08 slm for example. Through the purifier 20, the supplied oxygengas and nitrogen gas have suitable conditions for forming ozone and aresupplied to the ozone generator 19.

[0094] In the ozone generator 19, ozone gas is produced by plasma, forexample, acting on the supplied oxygen gas through a plasma generator.Then, a processing gas containing e.g. 2 to 18 vol. % of ozone issupplied from the ozone generator 19 through the processing gas supplypipe 18 to be blown upward in the cylindrical space D inside thereaction vessel 12 from the processing gas induction part 18 b.

[0095] The supply flow rate of the processing gas containing ozone shallbe satisfying to form a desired protective oxide film on the waferpositioned in the processing area PA and can be establishedcorresponding to different conditions in actuality: to cite instances,within the range approximately from 1 to 10 slm.

[0096] Also, for producing radical oxidative species, a method can beemployed wherein a hydrogen gas (H₂) and an oxygen gas (O₂) are supplieddirectly to the reaction process chamber PC at reduced pressure. In thismethod, for example, a hydrogen gas and an oxygen gas may be supplied ata ratio of 0.1:99.9 or 99.9:0.1 at a pressure of 13.3 to 133 Pa.

[0097] The above processing gas supply is accomplished under theconditions that the temperature inside the reaction process chamber PCis maintained at the temperature t2 for forming protective oxide filmsand the pressure is controlled as well.

[0098] The temperature t2 for forming protective oxide films is withinthe range or 25° C. (a room temperature) to 600° C. for example, lowerthan the temperature t3 for forming oxide films in the following processof oxide film formation in general. Although the range depends on thekinds or conditions of the means for forming protective oxide filmsactually employed, 50 to 550° C. is preferable and 200 to 400° C. inparticular is preferable.

[0099] This temperature t2 for forming protective oxide films may be thesame as or different from the above-mentioned temperature t1 forloading, but preferably the temperature t2 for forming protective oxidefilms is the same as the temperature t1 for loading in actualitybecause, in this case, problems accompanied with change of temperatureconditions such as causing defects in the formed protective oxide filmsfor example can be averted and also the protective-oxide-film-formingoperation can be started with high time efficiency as the establishedtemperature inside the reaction process chamber PC is not changed.

[0100] During the protective-oxide-film-forming operation, the pressureinside the reaction process chamber PC is maintained under depressurizedconditions, e.g. 13.3 to 101080 Pa (0.1 to 760 Torr).

[0101] Furthermore, this protective-oxide-film-forming operation isperformed within a controlled time frame. To be more precise, theprocessing gas is supplied into the reaction process chamber PC only fora controlled time period. This protective-oxide-film-forming operationtime is set for 1 to 130 minutes, for example.

[0102] In the above-mentioned protective-oxide-film-forming operation,oxygen atom radicals as radical oxidative species are produced due tothe activated ozone inside the reaction process chamber PC as ozone iscontained in the supplied processing gas, and since the oxygen atomradicals make contact with the wafer 16, the organic matters on thesurface of said wafer 16 are removed and also a protective oxide film isformed on the surface of said wafer 16.

[0103] Therefore, in this protective-oxide-film-forming operation, thefilm thickness of the formed protective oxide film will be thin withdense texture and also uniform since the temperature inside the reactionprocess chamber PC is maintained at the predetermined temperature t2 forforming protective oxide films and the pressure is maintained under thepredetermined depressurized conditions.

[0104] The film thickness of formed protective oxide films is 0.1 to 5nm for example, preferably 0.5 to 2 nm in particular.

[0105] In the shown oxidation treatment apparatus 10, the reactionvessel 12 has a single tubular structure in a state that the cylindricalspace D is defined between the inner peripheral wall of the reactionvessel 12 and the outer circumferential rim of the wafer 16 so thatpreferable exhaust conductance can be obtained which enables to maintainthe activated state of ozone stable and also maintain inside saidreaction process chamber PC under the predetermined depressurizedconditions. Furthermore, the preferable fact in improving theconductance inside the reaction process chamber PC is that theprocessing gas supply pipe 18 has at its end portion the processing gasinduction part 18 b bended and extended upward and also the vent 23 ispositioned at the opposite side of the processing gas supply pipe 18.

[0106] Moreover, a processing gas can evenly be supplied to the wafer 16in the processing area PA by supplying the processing gas upward towardthe cylindrical space D encircling the outer circumference of theprocessing area PA in which the wafer 16 is positioned. This is becausethe flow velocity of the processing gas is reduced in the processingarea PA and therefore has hardly any impact caused by the flow velocitydistribution at the blow into the reaction process chamber PC.

[0107] In the above-mentioned ozone generator 19, high ozone productionefficiency can be obtained by supplying a nitrogen gas as well as anoxygen gas. Moreover, although a processing gas will contain nitrogenoxides (NOx) due to the nitrogen gas supply, contamination inside thereaction process chamber PC due to corrosion of the processing gassupply pipe 18 can be avoided by using Teflon pipe material for theprocessing gas supply pipe 18 to obtain high resistance to corrosionagainst the contained nitrogen oxides.

[0108] (4) Heating Operation Inside the Reaction Vessel

[0109] The heating operation is performed on the wafer 16, on which aprotective oxide film is formed by the aboveprotective-oxide-film-forming operation, to allow for transfer to thefilm formation process which is an original purpose.

[0110] This heating operation may be proceeded to the competentcondition for the intended film formation process to be initiated.

[0111] Basically, heat is applied in this heating operation for insidethe reaction process chamber PC to reach the level of the requiredatmospheric condition for the film formation process, i.e. to reach thepreset temperature for the film formation process. The examples of theheating rate inside the reaction process chamber PC is not specificallylimited but within the range from 20 to 200° C./min., preferably 50 to150° C.

[0112] The pressure inside the reaction process chamber PC is controlledcorresponding to the preset pressure in the film formation process. Tobe more precise, the pressure is controlled to within 13.3 to 101080 Pa(0.1 to 760 Torr) and the partial pressure of oxygen is controlled toequal to or less than 1000 Pa for example, under the condition thatsupply of the processing gas containing ozone is ceased and an oxygengas or an oxygen gas and a nitrogen gas are supplied through the secondprocessing gas supply pipe 32 at a controlled flow rate.

[0113] In this heating process, although the temperature of a waferinside the reaction process chamber PC is gradually raised, the wafer isprotected by the protective oxide film formed on the surface of eachwafer, thereby proceeding without control of oxidization of wafers caneffectively be prevented.

[0114] By performing the above 4-step operations of (1) loadingoperation, (2) adjusting-processing-parameter operation, (3)protective-oxide-film-forming operation and (4) heating operation, aprotective oxide film is formed on the surface of a wafer inside thereaction process chamber PC under controlled conditions which the filmformation process can directly follow.

[0115] [Film Formation Process]

[0116] The film formation process is a primary process for the purposeof forming the intended oxide films on the above wafers.

[0117] According to the present invention, this film formation processis performed by providing an oxidation treatment wherein the temperatureof a wafer is raised under depressurized conditions in the atmosphereinside the reaction process chamber PC with oxygen.

[0118] Available means for the oxidation treatment are not specificallylimited, and the known conventional means for forming oxide films can beemployed without modification. The means to be employed can be selectedfrom or in combination of, for example, a wet oxidation method usingwater vapor, a dry oxidation method using ozone or other gasescontaining oxygen, an internal combustion method using a oxidizing gasobtained by combustion of proper gas inside the reaction processchamber, an oxidation method with radical oxidative species and anoxidation method with the atmosphere containing hydrogen chloride.

[0119] In this film formation process, the pressure inside the reactionprocess chamber may be within 133to 101080 Pa (1 to 760 Torr) dependingon the employed oxidation treatment means, and the temperature isgenerally set at the range from 750 to 1100° C.

[0120] An oxide film is formed on the surface (on the continued area ofa protective oxide film, to be exact) of a wafer through this filmformation process, and this oxide film offers quite high quality bybeing formed in the film formation process for the wafer on which aprotective oxide film is formed under the above-mentioned particularconditions, and further offers high uniformity in film thickness andfilm quality over the entire wafer even in a case of a thin film withfilm thickness of 5 nm for example.

[0121] Since an oxide film with dense texture and still high uniformityover the entire wafer despite the thinness in film thickness can beformed in this fashion according to the above-mentioned method, thepresent method for the formation of oxide films is effective as a methodfor forming gate-insulating films composing transistor chips, and saidgate-insulating films will have favorable characteristics and long life.

[0122] Moreover, by the heating operation performed as the finaloperation in the pretreatment process, the film formation process isallowed to be run in a state with a wafer still positioned inside thereaction process chamber PC after completion of the pretreatmentprocess, i.e. without removing the wafer out. Therefore, theprotective-oxide-film-forming operation is so-called an in-situoperation (in-situ: continuous processing with the same apparatus) fromthe viewpoint with a focus on the film formation process. Performing thefilm formation process following the pretreatment process in acontinuous manner in the same reaction process chamber PC in this way ishighly advantageous because oxide films are quite efficiently formed andwafers are not contaminated.

[0123] The film formation process is preferably performed by a dryoxidation method since the protective-oxide-film-forming operation isperformed with the processing gas containing ozone. In this case, manyof the facilities can be shared in the protective-oxide-film-formingoperation and the film formation process.

[0124] Although the conditions in the case of performing the filmformation process by a dry oxidation method are not specificallylimited, the processing may be performed for 1 to 300 minutes under theconditions that the temperature is 750 to 110020 C. and the pressure is133 to 101080 Pa (1 to 760 Torr), for example.

[0125] When the film formation process is completed, the pressurereturns to normal and the temperature is lowered to 25 to 600° C. insidethe reaction process chamber PC by supplying a predetermined amount ofnitrogen gas, e.g. for approximately 15 minutes at a flow rate of 20slm, through the nitrogen gas supply pipe 27. After that, the covermember 14 is brought down by actuating the boat elevator, and thusunloading is performed in the form the wafer boat 15 ejected from thereaction process chamber PC.

[0126] Hereinbefore described is the method for the formation of oxidefilms according to the present invention, and the oxidation treatmentapparatus employed for realizing the present invention is notspecifically limited.

[0127] Although the case of employing the batch-operating verticalapparatus for oxidation treatment structured as shown in FIG. 1 isdescribed above for example, the present invention can be realized withan oxidation treatment apparatus for single wafer processing, forexample. Also, the structure of-the reaction vessel etc. constructingthe reaction process chamber is limitless, and for example a double-pipestructure comprising an inner pipe and an outer pipe may be employed.

Embodiment

[0128] The embodiment of the present invention will be described belowalthough the present invention is not limited thereto.

[0129] Each of the following process was performed basically with theoxidation treatment apparatus with the structure shown in FIG. 1 andaccording to the sequence shown in FIG. 3.

[0130] [Precleaning Process]

[0131] The precleaning process was performed in a hydrofluoric acidsolution of 1 vol % concentration at a temperature of 23° C. byover-etching by 30% of the sacrificial oxide film of a silicon waferwith diameter of 8 inches.

[0132] [Pretreatment Process]

[0133] (1) Loading operation

[0134] The 100 pieces of wafers obtained by the precleaning process wereheld on the wafer boat 15 made of silicon dioxide which was then mountedon the cover member 14, and while the temperature was controlled at 300°C. and the pressure at 101080 Pa (760 Torr) inside the reaction processchamber PC with a nitrogen gas at a flow rate of 19.8 slm and an oxygengas at a rate of 0.2 slm being supplied to the reaction process chamberPC, said cover member 14 was inserted into the reaction vessel 12 at aroom temperature to position wafers 16 in the processing area PA.

[0135] (2) Adjusting-Processing-Parameter Operation The pressure insidethe reaction process chamber PC was reduced to 13.3 Pa or less by adepressurization operation, and the temperature inside the reactionprocess chamber PC was maintained at 300° C. as the preset temperaturefor forming protective oxide films t2 by controlling the warm-up heater17.

[0136] (3) Protective-Oxide-Film-Forming Operation

[0137] Then, as supply of a nitrogen gas from the nitrogen gas supplypipe 27 was ceased, the processing gas containing ozone at aconcentration of 14 volume percent was obtained from the ozone generator19 by supplying an oxygen gas from the oxygen gas supply pipe 2l at arate of 1 slm and supplying a nitrogen gas from the additive gas supplypipe 22 at a rate of 0.008 slm to the purifier 20, and this processinggas containing ozone was supplied into the reaction process chamber PCmaintained at a temperature of 300° C. and at a pressure of 26.6 Pa (0.2Torr) at a rate of 1 slm. This operation was continued for 9 minutes.

[0138] The analysis performed afterward confirmed that a protectiveoxide film with film thickness of 1 nm had been formed on each wafer 16by this protective-oxide-film-forming operation.

[0139] (4) Heating Operation Inside the Reaction Vessel

[0140] Following the above adjusting-processing-parameter operation, theheating operation was performed in a state that the wafers 16 were stillpositioned inside the reaction vessel 12.

[0141] To be more precise, the temperature inside the reaction processchamber PC was raised to 850° C. by actuating the warm-up heater 17 at aheating rate of 100° C./min while supply of the processing gascontaining ozone was ceased and an oxygen gas was supplied from thesecond processing gas supply pipe 32 at a rate of 0.5 slm.

[0142] [Film Formation Process]

[0143] Then, the film formation process was performed for 20 minutesunder the conditions that the temperature was 850° C. and the partialpressure of oxygen was 2660 Pa (20 Torr pressure) with the oxygen gassupply from the second processing gas supply pipe 32 maintained at therate of 0.5 slm.

[0144] After that, the pressure was returned to normal and thetemperature was lowered to 300° C. inside the reaction process chamberPC by supplying a nitrogen gas from the nitrogen gas supply pipe 27 at arate of 20 slm for 15 minutes, and then the wafer boat 15 was ejectedfrom the reaction process chamber PC by bringing the cover member 14down by actuating the boat elevator. Thus wafers on which oxide filmswith the respective total film thickness of 2.7 nm were manufactured.

[0145] The characteristics of the oxide films obtained in the above waywere examined with an NMOS capacitor prepared. That is to say, the rangeof the leakage current variation at an electric field intensity of −3MV/cm within the wafer was examined on each oxide film of the total of112 chip sections formed on one wafer to find out that the range waswithin 8% on the basis of the mean value, as shown in FIG. 4.

[0146] On the other hand, a protective oxide film with a mean thicknessof 0.8 nm was formed on a silicon wafer from the same lot by immersionin a hydrochloric acid solution at 60° C. for 10 minutes, the ratio ofH₂O₂:HCl:H₂O of the solution being 1:1:5. Then an oxide film was formedon this wafer to prepare a control sample by running the process ofoxide film formation (film formation process) by a dry oxidation methodunder exactly the same conditions as the above embodiment except for theprotective-oxide-film-forming operation, and the leakage currentvariation within the wafer was examined likewise to obtain the result of23% as shown in FIG. 4.

[0147] Based on the results stated above, the formed oxide film canobviously have high uniformity in film thickness and film quality overthe entire wafer according to the method for the formation of oxidefilms of the above-mentioned embodiment.

[0148] Also, reliability of the formed oxide films of the wafer obtainedin the above embodiment and the control sample were evaluated by theconstant voltage TDDB testing. FIG. 5 shows the results.

[0149]FIG. 5 illustrates the Weibull plot of TDDB data in which the plotgroup (A) shows the results of the above-mentioned embodiment and theplot group (B) shows the results of the above-mentioned control sample.The L is a boundary line between the plot groups (A) and (B).

[0150] In this case, the TDDB testing was performed under the conditionsthat the stress was maintained at −12.5 MV/cm and the chuck temperaturewas maintained at 120° C., and the capacitor area was 1×10⁻⁴ cm².

[0151] Based on the result shown in FIG. 5, the life as an insulatingfilm in the form of the oxide film of a wafer obtained in theabove-mentioned embodiment obviously is longer to a large extentcompared to the film in the form of the control sample.

[0152] In a case that the oxide films formed according to the method forthe formation of oxide films of the present invention are used asgate-insulating films composing transistor chips for example, it is tobe recognized that the transistor chips maintain the performance forextended period of time and attain high reliability.

[0153] As stated above, the quality oxide film with high uniformity infilm thickness and film quality over the entire area can be formed onthe surface of a wafer with advantage according to the method for theformation of oxide films of the present invention.

1. An oxide film formation method to form an oxide film on the surfaceof a wafer, the method being characterized by comprising: a pretreatmentprocess of forming a protective oxide film on the surface of a waferpositioned in a reaction vessel, the process being performed byoxidation treatment with radical oxidative species under depressurizedconditions or by oxidation treatment with an atmosphere containingradical oxidative species under depressurized conditions and a oxidefilm formation process to form an oxide film on the surface of saidwafer with the protective oxide film obtained by the pretreatmentprocess, the process being performed by oxidation treatment at apredetermined temperature under depressurized conditions.
 2. An oxidefilm formation method according to claim 1, characterized in that theoxide film formation process is performed following the pretreatmentprocess in a continuous manner in said reaction vessel in which saidpretreatment process is performed.
 3. An oxide film formation methodaccording to claim 1 or 2, characterized in that the pretreatmentprocess is performed at a temperature lower than the temperature for theoxide film formation process.
 4. An oxide film formation methodaccording to any one of claims 1 to 3, characterized in that thepretreatment process is performed under depressurized conditions, thelevel of the depressurization being higher than the level for the oxidefilm formation process.
 5. An oxide film formation method according toany one of claims 1 to 4, characterized in that the oxidation treatmentin the pretreatment process is performed under the conditions that thetemperature is from 25 to 600° C. and the pressure is from 13.3 to101080 Pa (0.1 to 760 Torr).
 6. An oxide film formation method accordingto any one of claims 1 to 5, characterized in that the radical oxidativespecies used for the oxidation treatment in the pretreatment process areproduced by ozone.
 7. An oxide film formation method according to anyone of claims 1 to 5, characterized in that the radical oxidativespecies are composed of O* (oxygen radical species) and OH* (hydroxylradical species).
 8. An oxide film formation method according to any oneof claims 1 to 7, characterized in that a wafer heating operation in thepretreatment process preceded to the oxide film formation process isperformed at a temperature from 750 to 1100° C. in an atmosphere with apartial pressure of oxygen of 1000 Pa or less.
 9. An oxide filmformation method according to any one of claims 1 to 8, characterized inthat the pressure is controlled to be from 13.3 to 101080 Pa (0.1 to 760Torr) and the partial pressure of oxygen is controlled to be 1000 Pa orless in the pretreatment process.
 10. An oxide film formation methodaccording to any one of claims 1 to 9, characterized in that theoxidation treatment in the oxide film formation process is performed bythe means selected from a wet oxidation method, a dry oxidation method,an internal combustion method, an oxidation method with radicaloxidative species and an oxidation method with an atmosphere containinghydrogen chloride, under the conditions that the pressure is from 133 to101080 Pa (1 to 760 Torr) and the temperature is from 750 to 1100° C.11. A gate-insulating film of transistor chip characterized in that thefilm is formed by the oxide film formation method according to any oneof claims 1 to
 10. 12. A method for forming gate-insulating films oftransistor chips characterized in that the method for the formation ofoxide films is performed according to any one of claims 1 to 10.